Wet friction material with pitch carbon fiber

ABSTRACT

A friction material includes a fibrous base material having at least one type of petroleum pitch-based carbon fiber.

CROSS REFERENCE TO RELATED APPLICATIONS

This divisional application claims the benefit of U.S. patentapplication Ser. No. 10/280,101 filed Oct. 24, 2002.

TECHNICAL FIELD

The present invention relates to a friction material comprising apetroleum pitch based carbon fiber. The friction material of the presentinvention has an increased coefficient of friction of about 20% comparedto standard carbon fibers. The friction material has extremely high heatresistance, improved durability and high temperature resistance.

BACKGROUND OF THE INVENTION

New and advanced continuous torque transmission systems, havingcontinuous slip torque converters and shifting clutch systems are beingdeveloped by the automotive industry. These new systems often involvehigh energy requirements. Therefore, the friction materials technologymust be also developed to meet the increasing energy requirements ofthese advanced systems.

In particular, a new high performance, durable friction material isneeded. The new friction material must be able to withstand high speedswherein surface speeds are up to about 65 m/seconds. Also, the frictionmaterial must be able to withstand high facing lining pressures up toabout 1500 psi. It is also important that the friction material beuseful under limited lubrication conditions.

The friction material must be durable and have high heat resistance inorder to be useful in the advanced systems. Not only must the frictionmaterial remain stable at high temperatures, it must also be able torapidly dissipate the high heat that is being generated during operatingconditions.

The high speeds generated during engagement and disengagement of the newsystems mean that a friction material must be able to maintain arelatively constant friction throughout the engagement periods. It isimportant that the frictional engagement be relatively constant over awide range of speeds and temperatures in order to minimize “shuddering”of materials during braking or the transmission system during powershift from one gear to another. It is also important that the frictionmaterial have a desired torque curve shape so that during frictionalengagement the friction material is noise or “squawk” free.

The principal performance concerns for all applications of the frictionmaterial are the prevention of shudder and the energy management of thefriction interface. The occurrence of shudder can be attributed to manyfactors including the friction characteristics of the friction material,the mating surface's hardness and roughness, oil film retention,lubricant chemistry and interactions, clutch operating conditions,driveline assembly and hardware alignment, and driveline contamination.The friction interface energy management is primarily concerned withcontrolling interface temperature and is affected by the pump capacity,oil flow path and control strategy. The friction material surface designalso contributes to the efficiency of interface energy management.

The main performance concerns for shifting clutch applications are thecoefficient of friction characteristics of the friction material (suchthat the friction material has a desired torque and holding capacity)and the stability of the friction material such that the frictionmaterial does not break down under use.

Previously, asbestos fibers were included in the friction material fortemperature stability. Due to health and environmental problems,asbestos is no longer being used. More recent friction materials haveattempted to overcome the absence of the asbestos in the frictionmaterial by modifying impregnating paper or fiber materials withphenolic or phenolic-modified resins. These friction materials, however,do not rapidly dissipate the high heat generated, and do not have thenecessary heat resistance and satisfactory high coefficient of frictionperformance now needed for use in the high speed systems currently beingdeveloped.

The Kearsey U.S. Pat. No. 5,585,166 describes a multi layer frictionlining having a porous substrate layer (cellulose and synthetic fibers,filler and thermoset resin) and a porous friction layer (nonwovensynthetic fibers in a thermoset resin) where the friction layer has ahigher porosity than the substrate layer.

The Seiz U.S. Pat. No. 5,083,650 reference involves a multi-stepimpregnating and curing process; i.e., a paper impregnated with acoating composition, carbon particles are placed on the paper, thecoating composition in the paper is partially cured, a second coatingcomposition is applied to the partially cured paper, and finally, bothcoating compositions are cured.

Various paper based fibrous materials have been developed that areco-owned by the assignee herein, BorgWarner Inc., for use in frictionmaterials. These and all the references disclosed herein are fullyincorporated herein by reference.

In particular, Lam et al., U.S. Pat. No. 5,998,307 relates to a frictionmaterial having a primary fibrous base material impregnated with acurable resin where the porous primary layer comprises at least onefibrous material and a secondary layer comprises carbon particlescovering at least about 3 to about 90% of the surface of the primarylayer.

The Lam et al., U.S. Pat. No. 5,858,883 relates to a base materialhaving a primary layer of less fibrillated aramid fibers, syntheticgraphite, and filler, and a secondary layer comprising carbon particleson the surface of the primary layer.

The Lam et al., U.S. Pat. No. 5,856,224 relates to a friction materialcomprising a base impregnated with a curable resin. The primary layercomprises less fibrillated aramid fibers, synthetic graphite and filler;the secondary layer comprises carbon particles and a retention aid.

The Lam et al. U.S. Pat. No. 5,958,507 relates to a process forproducing a friction material where about 3 to about 90% of at least onesurface of the fibrous material which comprises less fibrillated aramidfibers is coated with carbon particles.

The Lam, U.S. Pat. No. 6,001,750 relates to a friction materialcomprising a fibrous base material impregnated with a curable resin. Theporous primarily layer comprises less fibrillated aramid fibers, carbonparticles, carbon fibers, filler material, phenolic novoloid fibers, andoptionally, cotton fibers. The secondary layer comprises carbonparticles which cover the surface at about 3 to about 90% of thesurface.

Yet another commonly owned patent application Ser. No. 09/707,274relates to a paper type friction material having a porous primaryfibrous base layer with friction modifying particles covering about 3 toabout 90% of the surface area of the primary layer.

In addition, various paper type fibrous base materials are described incommonly owned BorgWarner Inc. Lam et al., U.S. Pat. Nos. 5,753,356 and5,707,905 which describe base materials comprising less fibrillatedaramid fibers, synthetic graphite and filler, which references are alsofully incorporated herein by reference.

Another commonly owned patent, the Lam, U.S. Pat. No. 6,130,176, relatesto non-metallic paper type fibrous base materials comprising lessfibrillated aramid fibers, carbon fibers, carbon particles and filler.

While self-stabilizing pitch and carbon fibers made therefrom have beendisclosed in various patents such as those assigned to Conaco Inc. U.S.Pat. Nos. 5,766,523; 5,540,903; 5,259,947; 5,437,780; 5,648,041;5,501,788; 5,540,832 and 6,123,829, all of which are expresslyincorporated herein by reference, none of these references teach ordisclose use of a carbon fibers of pitch carbon fibers in a wet frictionmaterial application.

For all types of friction materials, in order to be useful in “wet”applications, the friction material must have a wide variety ofacceptable characteristics. The friction material must have goodanti-shudder characteristics; have high heat resistance and be able todissipate heat quickly; and, have long lasting, stable and consistentfrictional performance. If any of these characteristics are not met,optimum performance of the friction material is not achieved.

It is also important that a suitable impregnating resin be used in thefriction material in order to form a high energy application frictionmaterial. The friction material must have good shear strength during usewhen the friction material is infused with brake fluid or transmissionoil during use.

Accordingly, it is an object of the present invention to provide animproved friction material with reliable and improved propertiescompared to those of the prior art.

A further object of this invention is to provide friction materials withimproved “anti-shudder”, “hot spot” resistance, high heat resistance,high friction stability and durability, and strength.

BRIEF DESCRIPTION OF THE DRAWINGS

The file of this patent contains at least one drawing executed in color.Copies of this patent with color drawing(s) will be provided by thePatent and Trademark Office upon request and payment of necessary fee.

FIG. 1 a is a SEM image of a cross-section of a pitch-based carbonfiber.

FIG. 1 b is a SEM image of a cross-section of a PAN base carbon fiber.

FIG. 2 is a graph showing test data for midpoint coefficient offriction, for Ex. 1-1, 1-2 and Compar. A.

FIG. 3 is a graph showing the ratio of endpoint (E) coefficient offriction to midpoint (M) coefficient of friction for Ex. 1-1, 1-2 andCompar. A.

FIG. 4 is a graph showing the breakaway coefficient of friction versusnumber of cycles for Ex. 1-1, 1-2 and Compar. A.

FIG. 5 is a graph showing the midpoint coefficient of friction overcycles 4, Ex. 2 and Compar. B.

FIG. 6 is a graph showing the E/M ratio for Compar. B and Ex. 2.

FIG. 7 is a graph showing the midpoint coefficient of friction for Ex. 2and Compar. B.

FIG. 8 is a graph showing piston displacement for Ex. 2 and Compar. B.

FIG. 9 is a graph showing the midpoint coefficient of friction for Ex.4-1, 4-2, 4-3 and 4-4 as compared to Compar. D-1, D-2, D-3 and D-4.

FIG. 10 is a graph showing a breakaway coefficient of friction for Ex.4-1, 4-2, 4-3 and 4-4 and Compar. Ex. D-1, D-2, D-3 and D-4.

FIG. 11 is a SEM photograph showing the fiber length of 20 microns forpetroleum pitch-based carbon fibers.

FIG. 12 is a SEM image showing 200 micron length petroleum pitch-basedcarbon fibers.

FIG. 13 is a cross-section f a petroleum pitch-based carbon fiber.

FIG. 14 is a graph showing the μPVT test for Ex. 5-1, 5-2 and Compar.E-1.

FIG. 15 a is a graph showing a T-N test data for level 8 of Ex. 5-1 andCompar. E-1 and E-2.

FIG. 15 b is an enlargement of the initial portion of the graph shown inFIG. 15A.

FIG. 16 is a T-N test showing piston displacement for Ex. 5-1 andCompar. E-1 and E-2.

FIG. 17 a is a graph showing the midpoint coefficient of friction forEx. 6-1, 6-2 and Compar. E-1.

FIG. 17 b is a graph showing the E/M ratio for Ex. 6-1, 6-2 and Compar.E-1.

FIG. 17 c is a graph showing the midpoint coefficient of friction forEx. 6-3, 6-4, 6-5a and 6-5b and Compar. F-1.

FIG. 17 d is a graph showing the E/M ratio for Ex. 6-3, 6-4, 6-5a and6-5b and Compar. F-1.

FIG. 18 a is a graph showing a T-N test data for low oil flow betweenthe midpoint coefficient of friction for Ex. 6-1, 6-2 and Compar. E-1.

FIG. 18 b is a graph showing the piston displacement for Ex. 6-1, 6-2and Compar. E-1.

FIG. 19 is a T-N test data showing at the various cycles for Ex. 6-1 and6-2 and Compar. E-1.

FIGS. 20 a and 20 b is a graph showing the T-N test data of Ex. 7-1,7-2, 7-3 and 7-4 and Compar. 3.

FIG. 21 is μVBPT test for Ex. 8-1, 82, 8-3, 8-4 and Compar. H-1.

FIG. 22 a is a graph showing the T-N data for the midpoint coefficientof friction for Ex. 8-1, 8-2, 8-3, 8-4 and Compar. H-1.

FIG. 22 b is a graph showing piston displacement for Ex. 8-1, 8-2, 8-3,8-4 and Compar. H-1.

FIG. 22 c is a μVPT test for Ex. 8-1, 8-2, 8-3, 8-4 and Compar. H-1.

SUMMARY OF THE INVENTION

The present invention relates to a friction material comprising afibrous base material having at least one type of petroleum pitch-basedcarbon fiber. The friction material is for use in engagement with anopposing friction surface. The fibrous base material has at least onetype of petroleum pitch-based carbon fiber which is present on at leastone outer surface of the friction material. Petroleum pitch-based carbonfiber is, thus, in contact with the opposing friction surface during theengagement of the friction material with the opposing friction surface.

In a preferred aspect the petroleum pitch-based carbon fiber comprises asolvated pitch which has a fluid temperature of at least 40° C. lowerthan the melting point of the same pitch in the nonsolvated state. Thepetroleum pitch-based fibers are capable of being heated tocarbonization temperatures without melting.

In certain preferred aspects the petroleum pitch-based carbon fibershave from about 5 to about 40% solvant, by weight. As such petroleumpitch-based carbon fibers are unmeltable upon the removal of the solvantfrom the fiber. Further advantages and objects of the present inventionare evident by referring to the following description and claims.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a friction material having petroleumpitch-based carbon fibers therein. In a preferred aspect, the petroleumpitch-based carbon fibers are present on at least one outer surface ofthe friction material. The petroleum pitch-based carbon fiber comes intocontact with an opposing friction surface during engagement of thefriction material.

The friction material of the present invention has a desired ratio ofendpoint (E) coefficient of friction to midpoint (M) coefficient offriction in the range of about 1 or less. The favorable E/M ratioindicates that the friction material of the present invention hasreduced shudder.

Further, the friction materials comprising the petroleum pitch-basedcarbon fibers have a midpoint coefficient of friction higher than theconventional friction materials. The midpoint coefficient of frictioncorrelates with the torque and holding capacity of the friction materialsuch that the friction material performs better than previous frictionmaterials.

In spite of the teachings of the art, it was surprisingly found that thepetroleum pitch-based carbon fibers did not cause a decrease in thecoefficient of friction, as would be expected with carbon materialssince carbon fibers are low friction materials. Rather, the coefficientof friction increased with an increase amount of petroleum pitch-basedcarbon fibers present in the friction material. Also, surprisingly, thefriction material having the petroleum pitch-based carbon fiber materialhas improved durability over comparative materials.

In certain embodiments, the petroleum pitch-based carbon fibers can beused in a single layer friction material.

In other embodiments, the petroleum pitch-based carbon fiber can be usedas a second layer on a fibrous base material which includes furtherpetroleum pitch-based carbon fibers in the fibrous base material.

In a yet another embodiment, the petroleum pitch-based carbon fibers areused as a secondary, or top layer on an outer surface of a fibrous basematerial that has no petroleum pitch-based carbon fibers in the fibrousbase material. The friction materials having the petroleum pitch-basedcarbon fibers as a secondary, or top, layer have increased durability.The petroleum pitch-based carbon fibers act as a heat shield and thusprovide additional desired stability to the friction material. Also, thedeposited petroleum pitch-based carbon fiber increases the break awaycoefficient of friction, thus increasing the holding capacity of thefriction material.

In certain embodiments, the petroleum pitch-based carbon fiber can beused as a top or secondary layer on an inexpensive porous materialcomprising, for example cotton and cellulose filler material.

It is to be understood that such carbon fiber materials can be presentin a typical formulation such as in the range of about 15 to about 20%,by weight. It is within the contemplated scope of the present inventionthat other suitable materials can be present in the fibrous basematerial. Such non-limiting examples include all foreseeable non-wovenmaterials including wet laid, dry laid, needle punch, knit, and stitchbonded non-woven materials. It is also within the contemplated scope ofthe present invention that other foreseeable forms of wet frictionmaterials can be used. It is also within the contemplated scope of thepresent invention that the petroleum pitch-based carbon fiber can beused alone or blended with other fibers and fillers.

The petroleum pitch-based carbon fiber has a unique structure. FIG. 1Ashows a cross-section of the pitch-based carbon fiber used in thepresent invention while FIG. 1B shows a cross-section of apolyacrylonitrile (PAN) based carbon fiber which is conventionally usedin friction materials.

In certain embodiments, the carbon fiber is made from a solvatedisotropic pitch which has a fluid temperature of at least about 40° C.lower, and often 200° C. or more, than the melting point of the samepitch in the non-solvated state. Fibers made from this solvatedisotropic pitch have desirable improved stabilization characteristicssuch that the fibers can be heated to carbonization temperatures withoutmelting. Further, any mesophase present in the carbon fibers is nothighly elongated by shear forces associated with the formation of thecarbon fibers. Further, preferred petroleum pitch-based carbon fibershave about 5 to about 40% solvent, by weight, where the pitch fiber isunmeltable upon removable of the solvent from the fiber.

In certain embodiments, the petroleum pitch-based carbon fiber used inthe present invention is a fiber (as described in U.S. Pat. No.6,123,829 fully incorporated herein by reference) which has an oxygendiffusion rate to its center which is approximate equal to, or greaterthan, the oxidation rate at an outer surface of the fibers. The fiber'scenter becomes oxidatably stabilized at a rate ranging from slightlyless than, to greater than, the rate of consumption of carbon by oxygenat the fiber's surface. The petroleum pitch-based carbon fiberspreferably have a softening point in excess of 300° C. and preferablygreater than 350° C. such that the fibers are capable of being subjectedto a stabilization process at temperatures greater than a fibrousspinning temperature. It was not until the present invention that suchpetroleum based carbon fibers were used in a friction material.

It is to be understood that “pitch” generally refers to by-products inthe production of natural asphalt petroleum pitches and heavy oilobtained as a by-product in a naphtha cracking industry and pitches ofhigh carbon content obtained from coal. Petroleum pitch generally refersto the residual carbonaceous material obtained from the catalytic and/orthermal cracking of petroleum distillates or residues. Solvated pitchesgenerally contain between about 5 to about 40% by wt. of solvant in thepitch and have a fluid temperature lower than the melting point of apitch component when not associated with solvent. Typically the fluidtemperature is lower than about 40° C. The fluid temperature for asolvated pitch is generally determined in the industry to be thetemperature at which the viscosity of 6000 poise is registered uponcooling the solvated pitch at 1° C. per minute from a temperature inexcess of its melting point. The solvant content refers to a valuedetermined by weight loss on vacuum separation of the solvent.

In certain aspects, the solvated pitch requires either a shorterstabilization step or no stabilization step during the production of thefibers, thus creating great cost savings for the stabilization. Usuallyoxidation has been needed in the past to produce other types of carbonfibers to prevent melting of the fibers when the fibers are heated tothe carbonization temperature. The petroleum pitch-based carbon fibersare defined herein as fibers following carbonization and/orgraphitization made from a solvated pitch. It is to be understood thatboth isotropic pitch (which comprises molecules not aligned in opticallyorder liquid crystal formation) and mesophase, or anisotropic, pitch(which comprises molecules having aromatic structures, which throughinteraction, are associated together to form optically ordered liquidcrystal which are either liquid or solid depending on the temperature)are useful in making the petroleum pitch-based carbon fibers used in thepresent invention.

EXAMPLES

The following examples illustrate various embodiments of the presentinvention. It should be understood, however, that other embodiments notshown in the following examples are also contemplated as being withinthe scope of the present invention.

Example I

In certain embodiments, the friction material can comprise a fibrousbase material comprising, by weight percent: about 20 to about 60%fibrillated aramid fibers, about 10 to about 30% silica filler material,about 10 to about 20% graphite, and about 5 to about 20% petroleumpitch-based carbon fibers. In one embodiment of the present invention,Ex. 1-1 and 1-2 comprise fibrillated aramid fiber at about 50%, silicafiller at about 20%, graphite at about 15%, petroleum pitch-based carbonfiber at about 15%, and optionally latex at about 2% addon was used tomake a fibrous base material having a basis weight of about 155 lb/3000ft² and having a caliper of about 29 mils.

FIG. 2 is a test showing a shifting clutch application showing amidpoint coefficient of friction for two examples of petroleumpitch-based material (shown as Ex. 1-1 and Ex. 1-2) and a comparativestandard carbon fiber (shown as Compar. 1) for different cycles, labeledA through P. According to the present invention, it is desired to havethe friction material have a ratio of endpoint (E) coefficient offriction to midpoint (M) coefficient of friction of about 1 or less.This E/M ratio is shown in FIG. 3 where the friction material comprisingpetroleum pitch-based carbon fibers generally have an E/M ratio lessthan E/M ratio for the comparative friction material having the fibermaterial.

The breakaway coefficient of friction test shown in FIG. 4 depicts theholding capacity of the friction material. As can be seen, the holdingcapacity for the Ex. 1 friction materials comprising petroleum-basedcarbon fibers is much higher than for the conventional fiber. Thefriction materials comprising the petroleum pitch-based carbon fibershave a midpoint coefficient of friction about 20% higher than theCompar. A conventional friction materials having standard carbon fibers.Further the Ex. 1 materials have a desirable low end to midpointcoefficient of friction ratio and have a breakaway coefficient offriction of about 11 to 19% higher than the conventional material.

Example II

In another embodiment of the present invention, the friction materialcomprises by weight percent, about 50 to about 60% aramid fibers, about3 to about 10% silica filler, about 20 to about 30% graphite and about10 to about 20% petroleum pitch-based carbon fibers. In anotherembodiment of the present invention, Ex. 2 is a friction material havingthe following formulation, by weight:

-   Aramid fibers, about 55%,-   Silica filler, about 5%,-   Graphite, about 25%, and-   Petroleum pitch-based carbon fiber, about 15%; and optionally-   Latex addon about 2% additional.

The Ex. 2 fibrous base material has a basis weight of about a 166lb/3000 ft² basis weight and a caliper of about 29 mils.

FIG. 5 shows the test for the midpoint coefficient of friction for theprimary layer for a Compar. B comparative sample which contains asimilar formulation to Ex. 2 except that conventional carbon fibers wereused instead of petroleum pitch-based carbon fibers.

The endpoint/midpoint ratio for the primary layers of Ex. 2 and Compar.B are shown in FIG. 6. The materials stay in desirable ranges.

FIG. 7 is a graph of test data showing midpoint coefficient of frictionfor the primary layers for the carbon fiber in Ex. 2 and the Compar. B.

FIG. 8 is a graph of a test comprising piston displacement to cycles.The Ex. 2 petroleum based carbon fiber material performs the same as theCompar. B.

Example III

In yet another embodiment of the present invention the friction materialcomprises a fibrous base material comprising, by weight percent, about50 to about 70% silica friction modifying materials and about 30 toabout 50% fibers where the fibers comprise a mixture of aramid fibersand petroleum pitch-based carbon fibers. In certain embodiments, thepetroleum pitch-based carbon fibers are present at about 5 to about 50%of the fibers present in the friction material, and in certainembodiments, the petroleum pitch-base fibers are present at about 45 toabout 55% of the fibers.

In other embodiments, the amount of petroleum pitch-based carbon fiberscan be varied, depending on the end use application. As such, variousfriction materials have about 45 to about 55% petroleum pitch-basedfibers based on the amount of fibers present in the friction materialwhile others have about 30 to about 40% petroleum pitch-base fibers;still others have about 10 to about 20% petroleum pitch-based carbonfibers; and still others have about 3 to about 7% petroleum pitch-basedcarbon fibers. In the following examples, the friction materialscomprise a celite and fibrous base material having different ratios ofaramid fibers to petroleum pitch-based carbon fibers. The comparativeexamples contain fibrous base materials having conventional carbonfibers.

When the formulations contain an increased amount of petroleumpitch-based carbon fibers, there was an expectation that the coefficientof friction would drop since the carbon fiber is a low frictionmaterial. Surprisingly, however, the coefficient of friction increasedwith an increase in the amount of petroleum pitch-based carbon fiberpresent in the invention. The following examples were tested.

TABLE 1 Compar. E-5 Compar. F-1 Material Range % Range % Cotton Fibers10–15 15–20 Aramid Fibers 35–45 35–45 Carbon Fiber  3–10  3–10 CarbonParticles 10–30 10–20 Celite filler 25–35 15–20 Latex addon

FIG. 9 shows the petroleum pitch-based carbon fiber μ PVT test in DEXIII for samples different percents of carbon fiber. FIG. 9 clearly showsthat the midpoint coefficient of friction is greater overall for theExamples having petroleum pitch-based carbon fibers and, mostsurprisingly, is greatest at the highest concentrations of petroleumpitch-based carbon fibers.

The break-away coefficient of friction test for these Ex. 4-1, 4-2, 4-3,and 4-4 and Compar. D-1, D-2, D-3 and D-4 are shown in FIG. 10. Thus, itcan be clearly seen that increasing amounts of standard carbon fiber,decreases the coefficient of friction while, surprisingly, thecoefficient of friction in the friction materials with the petroleumpitch-based carbon fiber remains steady or increased.

Example IV

In yet another embodiment of the present invention, the petroleumpitch-base carbon fiber is deposited on a fibrous base material havingno petroleum pitch-base carbon fibers in the fibrous base material.

The primary layer can comprise different types of fibrous basematerials. One useful fibrous based material comprises, by weightpercent, about 10 to about 15% cotton fibers, about 35 to about 45%aramid fibers, about 3 to about 10% non-petroleum pitch-based carbonfibers, about 10 to about 20% carbon particles, and about 25 to about35% celite friction modifying materials.

Another primary layer useful with the present invention comprises, byweight percent, about 5 to about 20% cotton fibers, about 10 to about50% aramid fibers, about 10 to about 35% carbon particles, and about 2to about 15% non-petroleum pitch-based carbon fibers.

In certain embodiments, the petroleum pitch-based carbon fibers can havean average length of about 10 to about 30 microns. In other embodiments,the petroleum pitch-based carbon fibers can have an average length ofabout 150 to about 250 microns.

It is to be understood that the amount of petroleum pitch-based carbonfibers can range from about 1 to about 15 lbs/3000 sq. ft. Particularlyuseful embodiments include a secondary layer of about 3 to about 5, andmost preferably about 4 lbs/3000 sq. ft. Still other useful embodimentsinclude about 8 to about 15 lbs/3000 sq. ft. In certain embodiments, thepetroleum pitch-based carbon fiber secondary layer can be combined withcelite friction modifying materials having an average diameter of about6 microns. In the example herein, about 9 lbs./3,000 sq. ft. ofpetroleum pitch-base carbon fibers, (in some examples having lengths ofabout 20 microns and in other examples having average lengths of about200 microns), were deposited on samples of fibrous base materials asshown in Table 2 below.

TABLE 2 Example Ex. Compar. Ex. Compar. 4-1 D-1 4-2 D-2 Carbon % 49.8 4534.2 30 Fiber BW #/3000 160 160 160 160 Caliper In 0.0279 0.0291 0.0270.0281 Density G/cc 0.367 0.352 0.380 0.365 Example Ex. Compar. Ex.Compar. 4-3 D-3 4-4 D-4 Carbon % 17.7 15 6 5 Fiber BW #/3000 160 160 160160 Caliper In 0.0282 0.029 0.0287 0.0289 Density G/cc 0.364 0.353 0.3570.355

FIGS. 11, 12 and 13 show microphotographs of the material used in thisexample. FIG. 11 shows Ex. 5-1 which has a top or secondary layer ofpetroleum pitch-based carbon fibers having the average length of about20 microns. FIG. 12 shows Ex. 5-2 which has a top, or secondary layer ofpetroleum pitch-based carbon fibers having an average length of about200 microns. FIG. 13 is a cross-sectional view of the fibers. In eachexample, about 9 lbs. of the carbon fiber were deposited on the fibrousbase materials.

FIG. 14 shows the μPVT test for the Compar. E material.

FIGS. 15 a and 15 b show a T-N test showing a step level test for thefriction materials for Compar. E-1 and Compar. E-2 compared to Ex. 5-1of the present invention. As can be seen in FIGS. 15 a and 15 b, thepetroleum pitch-base carbon fiber material improves the durability ofthe friction material. Further, it should be understood that, in certainembodiments, 9 lbs. deposit can be greatly decreased while retaining thedesired durability and friction characteristics. Note that Compar. E-2(BWA 6100) is similar to the Compar. E-1 which has 9 lbs. of celitedeposited on the surface. Therefore, a direct comparison of 9 lbs. ofsecondary material can readily be made between looking at Ex. 5-1 andCompar. E-2.

Further, the step durability test T-N shown at level 8 in FIG. 16 for apiston displacement test compares the Compar. E-2, Compar. E-1 with theEx. 5-1. There is less overall piston displacement and therefore morestability for the friction materials having the petroleum pitch-basecarbon fiber as a secondary or top layer. The petroleum pitch-basedcarbon fibers acts as a heat shield and thus provides stability.

Example V

In yet another embodiment of the present invention, lower amounts ofpetroleum pitch-based carbon fiber (pp-carbon) material were depositedon the Compar. E-1 and a Compar. F-1 whose formulations are given below.

TABLE 3 The Examples in FIGS. 17a–d comprising: Base Layer SecondaryLayer Ex. 6-1 Compar. E-5  4 lb. 20 μm pp-carbon fiber Ex. 6-2 Compar.E-5  4 lb. 20 μm pp-carbon fiber and  9 lb. smaller sized celite fillerEx. 6-3 Compar. F-1  4 lb 20 μm pp-carbon fiber Ex. 6-4 Compar. F-1  9lb 20 μm pp-carbon fiber Ex. 6-5a Compar. F-1  9 lb 20 μm pp-carbonfiber and 12 lb 6μ ave. diameter celite filler with latex addon Ex. 6-5bCompar. F-1  9 lb 20 μm pp-carbon fiber and 12 lb 6μ ave. diametercelite filler Compar. E-5 Cotton 10–15% 0 Aramid 35–45% Carbon fiber3–10% Carbon particles 10–30% Celite 25–35% Compar. F-1 Cotton 15–20% 0Aramid 35–40% Carbon Fiber 3–10% Carbon particles 10–20% Celite 15–20%

In various examples the petroleum pitch-based carbon fibers weredeposited alone and, in other examples in combination with either 9 lbs.or 12 lbs. of a superfloss-type Celica filler which has an averagemicron particle size of about 6 microns which is smaller than the celitefriction material particle size.

FIGS. 17 a-d compare the examples and show that, while the effect of theadditional petroleum pitch-based carbon fiber material on thecoefficient of friction is minimal, the deposited petroleum pitch-basedcarbon fiber increases the break-away coefficient of friction.

FIGS. 18 a and 18 b compare the commercial product, Compar. E-1, withEx. 6-1 and Ex. 6-2. When a lower amount, such as the 4 lbs. ofpetroleum pitch-based carbon fiber deposit is used as a deposit orsecondary layer, the coefficient of friction is slightly higher than theCompar. E-1 base material while the durability is of the Ex. 6-1improved significantly.

FIGS. 19 a, b and c show T-N test data at 400 cycles. The petroleumpitch-based carbon fiber, when used as a secondary layer, either aloneor in combination with silica filler materials, provide functionmaterials that have improved durability.

Example VI

In yet another embodiment of the present invention, petroleum pitch-basecarbon fibers (pp-carbon fiber) also improve the properties of a furthercommercial product, Compar. G, which is an extremely porous fibrous basematerial comprising about 80% fiber and about 20% filler which has avery open structure.

TABLE 4 Base Layer Secondary Layer Ex. 7-1 Compar. G 4 lb pp-carbonfiber Ex. 7-2 Compar. G 9 lb pp-carbon fiber Ex. 7-3 Compar. G 9 lbpp-carbon fiber and 12 lb 6μ ave. diameter celite filler Ex. 7-4 Compar.G 9 lb pp-carbon fiber and 12 lb 6μ ave. diameter celite filler withlatex addon Compar. G 20% filler 80% fiber

FIGS. 20 a and 20 b show for the T-N data for Compar. G and Ex. 7-1 to7-4.

Ex. 7-1 with the 4 lbs. of the petroleum pitch-base carbon fiber deposithas a slightly higher the desirable coefficient of friction than theCompar. G base material alone. Further, the durability of the examplesof the present invention was greatly increased.

Example VII

In yet another embodiment of the present invention, various formulationswere made using mixed deposits, or secondary layers. A differentcommercial product H, comprises a fibrous base layer material which isalso porous, and comprises about 80% fiber and about 20% filler and hasan open structure.

The following examples were made:

TABLE 5 Base Layer Secondary Layer Compar. H-1 Compar. H 18 lb 6μ ave.diameter celite filler Ex. 8-1 Compar. H 18 lb 6μ ave. diameter celitefiller;  2 lb, 20 μm pp-carbon fiber Ex. 8-2 Compar. H  9 lb 6μ ave.diameter celite filler;  2 lb 20 μm pp-carbon fiber Ex. 8-3 Compar. H  9lb 6μ ave. diameter celite filler;  2 lb 20 μm pp-carbon fiber Ex. 8-4Compar. H  2 lb 6μ ave. diameter celite filler;  2 lb 20 μm pp-carbonfiber Compar. H-2 Compar. H 18 lb symmetrically shaped frictionparticles.

FIG. 21 show that the friction materials with petroleum pitch-basecarbon fiber have no detrimental effect on the μVPT tests and that thefriction level remains the same; that is, FIG. 21 shows a positivedownward slope with no undesirable rooster tail configuration.

FIGS. 22 a, 22 b and 22 c show the T-N test for low oil flow at 0.4l/minutes. All samples showed similar coefficients of friction material.It is to be noted that Ex. 8-4 shows the best durability while Ex. 8-2also showed excellent durability. These examples show that a smallamount of the petroleum pitch-base carbon fiber material present as asecondary layer is sufficient to improve durability of the frictionmaterial.

The friction material of the present invention can be impregnated withvarious types of resin formulations. In the past, the followingformulations have been found to be useful. However, it is to beunderstood that it is within the contemplated scope of the presentinvention that other resin formulations can be used in the presentinvention.

Various types of friction modifying particles are useful in the frictionmaterial. In one embodiment, useful friction modifying particles includesilica particles. Other embodiments can have friction modifyingparticles such as resin powders such as phenolic resins, silicone resinsepoxy resins and mixtures thereof. Still other embodiments can includepartial and/or fully carbonized carbon powders and/or particles andmixtures thereof; and mixtures of such friction modifying particles. Incertain embodiments, silica particles such as diatomaceous earth,Celite®, Celatom®, and/or silicon dioxide are especially useful. Thesilica particles are inexpensive inorganic materials which bond stronglyto the base material. The silica particles provide high coefficients offriction to the friction material. The silica particles also provide thebase material with a smooth friction surface and provide a good “shiftfeel” and friction characteristics to the friction material such thatany “shudder” is minimized.

In certain embodiments, the friction material can be impregnated usingdifferent resin systems. In certain embodiments, it is useful to use atleast one phenolic resin, at least one modified phenolic-based resin, atleast one silicone resin, at least one modified silicone resin, at leastone epoxy resin, at least one modified epoxy resin, and/or combinationsof the above. In certain other embodiments, a silicone resin blended ormixed with a phenolic resin in compatible solvents is useful.

Various resins are useful in the present invention. In certainembodiments, the resin can comprise phenolic or phenolic based resins,preferably so that the saturant material comprises about 45 to about 65parts, by weight, per 100 parts, by weight, of the friction material.After the resin mixture has been applied to the fibrous base materialand the fibrous base material has been impregnated with the resinmixture, the impregnated fibrous base material is heated to a desiredtemperature for a predetermined length of time to form a frictionmaterial. In certain embodiments, the heating cures the phenolic resinpresent in the saturant at a temperature of about 300° F. When otherresins are present in the saturant, such as a silicone resin, theheating cures the silicone resin at a temperature of about 400° F.Thereafter, the cured friction material is adhered to a desiredsubstrate by suitable means.

Various useful resins include phenolic resins and phenolic-based resins.It is to be understood that various phenolic-based resins which includein the resin blend other modifying ingredients, such as epoxy,butadiene, silicone, tung oil, benzene, cashew nut oil and the like, arecontemplated as being useful with the present invention. In thephenolic-modified resins, the phenolic resin is generally present atabout 50% or greater by weight (excluding any solvents present) of theresin blend. However, it has been found that friction materials, incertain embodiments, can be improved when the mixture includes resinblend containing about 5 to about 80%, by weight, and for certainpurposes, about 15 to about 55%, and in certain embodiments about 15 toabout 25%, by weight, of silicone resin based on the weight of thesilicone-phenolic mixture (excluding solvents and other processingacids).

Examples of useful phenolic and phenolic-silicone resins useful in thepresent invention are fully disclosed in the above-referenced BorgWarnerU.S. patents which are fully incorporated herein, by reference. Siliconeresins useful in the present invention include, for example, thermalcuring silicone sealants and silicone rubbers. Various silicone resinsare useful with the present invention. One resin, in particular,comprises xylene and acetylacetone (2,4-pentanedione). The siliconeresin has a boiling point of about 362° F. (183° C.), vapor pressure at68° F. mm, Hg: 21, vapor density (air=1) of 4.8, negligible solubilityin water, specific gravity of about 1.09, percent volatile, by weight,5% evaporation rate (ether=1), less than 0.1, flash point about 149° F.(65° C.) using the Pensky-Martens method. It is to be understood thatother silicone resins can be utilized with the present invention. Otheruseful resin blends include, for example, a suitable phenolic resincomprises (% by wt.): about 55 to about 60% phenolic resin; about 20 toabout 25% ethyl alcohol; about 10 to about 14% phenol; about 3 to about4% methyl alcohol; about 0.3 to about 0.8% formaldehyde; and, about 10to about 20% water. Another suitable phenolic-based resin comprises (%by wt.): about 50 to about 55% phenol/formaldehyde resin; about 0.5%formaldehyde; about 11% phenol; about 30 to about 35% isopropanol; and,about 1 to about 5% water.

It has also been found that another useful resin is an epoxy modifiedphenolic resin which contains about 5 to about 25 percent, by weight,and preferably about 10 to about 15 percent, by weight, of an epoxycompound with the remainder (excluding solvents and other processingaids) phenolic resin. The epoxy-phenolic resin compound provides, incertain embodiments, higher heat resistance to the friction materialthan the phenolic resin alone.

In certain embodiments, it is preferred that resin mixture comprisesdesired amounts of the resin and the friction modifying particles suchthat the target pick up of resin by the fibrous base material rangesfrom about 25 to about 70%, in other embodiments, from about 40 to about65%, and, in certain embodiments, about 60 to at least 65%, by weight,total silicone-phenolic resin. After the fibrous base material issaturated with the resin, the fibrous base material is cured for aperiod of time (in certain embodiments for about ½ hour) at temperaturesranging between 300-400° C. to cure the resin binder and form thefriction material. The final thickness of the friction material dependson the initial thickness of the fibrous base material.

It further contemplated that other ingredients and processing aids knownto be useful in both preparing resin blends and in preparing fibrousbase materials can be included, and are within the contemplated scope ofthe present invention.

In certain embodiments, the resin mixture can comprise both the siliconeresin and the phenolic resin which are present in solvents which arecompatible to each other. These resins are mixed together (in preferredembodiments) to form a homogeneous blend and then used to saturate thefibrous base material. In certain embodiments, there is not the sameeffect if the fibrous base material is impregnated with a phenolic resinand then a silicone resin is added thereafter or vice versa. There isalso a difference between a mixture of a silicone-phenolic resinsolution, and emulsions of silicone resin powder and/or phenolic resinpowder. When silicone resins and phenolic resins are in solution theyare not cured at all. In contrast, the powder particles of siliconeresins and phenolic resins are partially cured. The partial cure of thesilicone resins and the phenolic resins inhibits a good saturation ofthe base material.

In certain embodiments of the present invention, the fibrous basematerial is impregnated with a blend of a silicone resin in a solventwhich is compatible with the phenolic resin and its solvent. In oneembodiment, isopropanol has been found to be an especially suitablesolvent. It is to be understood, however, that various other suitablesolvents, such as ethanol, methyl-ethyl ketone, butanol, isopropanol,toluene and the like, can be utilized in the practice of this invention.The presence of a silicone resin, when blended with a phenolic resin andused to saturate the fibrous base material, causes the resultingfriction materials to be more elastic than fibrous base materialsimpregnated only with a phenolic resin. When pressures are applied tothe silicone-phenolic resin blended impregnated friction material of thepresent invention, there is a more even distribution of pressure which,in turn, reduces the likelihood of uneven lining wear. After thesilicone resin and phenolic resin are mixed together with the frictionmodifying particles, the mixture is used to impregnate the fibrous basematerial.

The friction material of the present invention includes a layer offriction modifying particles on a top surface of a fibrous base materialprovides a friction material with good anti-shudder characteristics,high resistance, high coefficient of friction, high durability, goodwear resistance and improved break-in characteristics.

1. A friction material comprising a fibrous base material comprising, byweight percent: about 50 to about 60% aramid fibers; about 3 to about10% silica filler; about 20 to about 30% graphite; about 10 to about 20%petroleum pitch-based carbon fibers; and, a secondary layer comprisingcarbon particles and a silica filler material.
 2. A friction materialcomprising a fibrous base material comprising, by weight percent: about50 to about 60% aramid fibers; about 3 to about 10% silica filler; about20 to about 30% graphite; about 10 to about 20% petroleum pitch-basedcarbon fibers; and a secondary layer comprising carbon particles and asilica filler material, wherein the carbon particles are present atabout 1–3 lbs. and the silica friction material is present at about 2–4lbs. based on a basis weight of lbs/3000 square feet.
 3. The frictionmaterial of claim 1, wherein the petroleum pitch-based carbon fiber ismade from a solvated pitch which has a fluid temperature of at leastabout 40° C. lower than the melting point of the same pitch in thenonsolvated state wherein the petroleum pitch-based fibers are capableof being heated to carbonization temperatures without melting.
 4. Thefriction material of claim 3, wherein the petroleum pitch-based carbonfibers have from about 5 to about 40% solvent, by weight, wherein thepitch fiber is unmeltable upon removal of the solvent from the fiber. 5.The friction material of claim 3, wherein the petroleum pitch-basedcarbon fiber has an oxygen diffusion rate to a center of the fiber whichis approximately equal to, or greater than an oxygen rate at an outersurface of the fiber.
 6. The friction material of claim 4, wherein thepetroleum pitch-based carbon fibers have a softening point of at leastabout 300° C.
 7. A friction material comprising a primary layercomprising a fibrous base material and a secondary layer comprising ofpetroleum pitch-based carbon fibers deposited on the primary layer, thepetroleum pitch-based carbon fibers having an average length of about 10to about 150 microns.
 8. A friction material comprising a primary layercomprising a fibrous base material and a secondary layer deposited onthe primary layer, comprising petroleum pitch-based carbon fibers thatare present at about 9 lbs. based on a basis weight of lbs./3000 squarefeet; the petroleum pitch-based carbon fibers having an average lengthof about 150 to about 250 microns.
 9. The friction material of claim 7,wherein the primary layer comprises, by weight %: about 10 to about 15%,cotton fibers; about 35 to about 45% aramid fibers; about 3 to about 10%non-petroleum pitch-based carbon fibers; about 10 to about 20% carbonparticles, and about 25 to about 35% celite friction modifyingmaterials.
 10. The friction material of claim 7, wherein the primarylayer comprises, by weight %: about 5 to about 20% cotton fibers; about10 to about 50% aramid fibers; about 10 to about 35% carbon particles;and about 2 to about 15% non-petroleum pitch-based carbon fibers. 11.The friction material of claim 7, wherein the petroleum pitch-basedcarbon fiber is made from a solvated pitch which has a fluid temperatureof at least about 40° C. lower than the melting point of the same pitchin the nonsolvated state wherein the petroleum pitch-based fibers arecapable of being heated to carbonization temperatures without melting.12. The friction material of claim 7, wherein the petroleum pitch-basedcarbon fibers have from about 5 to about 40% solvent, by weight, whereinthe pitch fiber is unmeltable upon removal of the solvent from thefiber.
 13. The friction material of claim 7, wherein the petroleumpitch-based carbon fiber has an oxygen diffusion rate to a center of thefiber which is approximately equal to, or greater than an oxygen rate atan outer surface of the fiber.
 14. The friction material of claim 7,wherein the petroleum pitch-based carbon fibers have a softening pointof at least about 300° C.